Spatial distribution and recent changes in carbon, nitrogen and phosphorus accumulation in sediments of the Black Sea

Sediment cores from the Black Sea were analyzed along two transects across the basin in West–East and North–South direction and ranging from the oxic and suboxic shelf to the anoxic slopes and abyssal plain. On the North-Western shelf, the average concentrations of total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) in recent sediments showed a decrease with increasing distance to the shore. Compared to the shelf, TP concentration was depleted on the slope and towards the central basin whereas TOC and TN concentrations increased towards the deep central basin. Anthropogenic nutrient loads of the last 50 years have left a clear signature in the sedimentary record. On the shelf, TN and TP were about 40% and 10% higher, respectively, than in the period 1850–1950. Anthropogenic impact on deep-sea sediments is within the natural variability for the last 50 years (+ 5% TN and + 8% TP). Our data and a literature survey were the basis for identifying three major sedimentary areas and for estimating the total annual accumulation of 1.3 × 106 t TOC, 1.4 × 105 t TN and 4.7 × 104 t TP. A mass balance based on river inputs, outflow to the Sea of Marmara and the total accumulation rates indicated that 20% of the TN inputs accumulated in the sediments of the Black Sea whereas denitrification eliminated more than 55% of the inputs. In contrast, a single removal process controls the TP budget with the sedimentary accumulation, mainly on the shelf, representing 80% of the total incoming load.